A perpetual goal in digital circuitry is to maintain pulse fidelity and minimize settling time. For digital circuitry exchanging high-speed data, e.g., such as with the Fibre Channel standard, it is increasingly the case that even circuit traces of short lengths are a significant fraction of a wavelength long. In order to get a clean transient response for the high-speed data, such short traces and the nearby ground planes are treated as transmission lines whose impedances should be matched to the circuitry and/or to a standardized nominal value.
Depending on the type of digital circuitry and the needs of the circumstances to which the transmission lines are applied, a variety of techniques may be employed to achieve an impedance match or a nominal impedance. These include damping resistors, source end series termination, load-end termination, distributed loading, etc.
Such high-speed signals place the greatest demand upon a connection-plane structure, e.g., a midplane or backplane, to which data-handling units connect. A midplane is a PCB designed to interconnect other PCBs (daughter cards) or modules (such as power supplies or disk drives). It is typically positioned in a product such that other boards or modules may connect from either side (front or back). In contrast, a backplane provides the same type of functionality, but typically only accepts boards or modules from one side. Examples of a data-handling unit include: data-storage modules, input/output (I/O) modules, power supply modules, cooling (such as fan or blower) modules and other field replaceable units (FRUs).
The connection-plane structure mechanically supports the data-handling units that are mechanically-connected to it. The connection-plane structure also provides electrical signal paths (formed of conductors or traces) between data-handling units by which high-speed data can be exchanged. Power can be drawn from the connection-plane structure. Status and/or control signals can be exchanged with a data-handling unit via the connection-plane structure. Alternatively, the electrical signal paths between data-handling units can be provided via a cable harness.
When taking the form of the connection-plane structure or a cable harness having a matched and/or nominal impedance, electrical signal paths between data handling units introduce significant costs and add significant complexity to a system.